Total protein synthesis in splenic microsomes isolated from non-immunized riboflavin and pantothenic acid-deficient rats was not significantly different from that of control rats. However, in rats immunized with Bordetella pertussis, total protein synthesis by isolated splenic and lymph node microsomes from the deficient animals was less than that of controls. In contrast, pantothenic acid deficiency in immunized rats affected neither the ability of intact lymph node and splenic cells to synthesize protein or immunoglobulins nor to secrete these compounds into the extracellular medium. The weights of lymph nodes and the number of cells per gram of lymph node tissue were diminished in pantothenic acid-deficient immunized rats. We plan to investigate the effects of specific deficiencies of riboflavin, pyridoxine and pantothenic acid upon the antibody-forming cells. These studies will encompass the enumeration of T and B cells and their interactions in the production of the immune response.